In the conventional injection molding of plasticized synthetic resin by an injection molder, the weight of the injected plasticized synthetic resin varies with the molten resin pressure, the molten resin specific volume or the molten state of the resin indicated by the molten resin temperature etc. (including the influence of the disturbance on the molding system). Therefore, it is difficult to form products of a constant quality. To cope with this problem, a number of proposals, so-called adaptive control, have been made. The art disclosed in the Japanese Patent Laid Open Publication No. 84932 of Showa 56 (the year 1981) is an example.
Basically, in the adaptive control method, when the influence of a disturbance on the molding system and change in the molten resin pressure, molten resin temperature or mold temperature are detected, a controllable molding condition (pressure, time, etc.) other than the detected factors are changed into a control factor, thereby assuring the constant product quality.
However, this control method has the following disadvantages:
1. Preliminary study and analysis are necessary to determine the correlation between the detected factor according to the mold and the product quality and between the control factor and the product quality. PA0 2. With the same plasticized synthetic resin, the correlation between the detected factor and the product quality and between the control factor and the product quality varies greatly depending on the mold. Therefore, the study and analysis described in the item 1 above must be repeated each time the mold is changed.
Injection molding process is generally described in view of the plasticized synthetic resin property such as the PVT property [molten resin pressure-molten resin specific volume-molten resin temperature (=molten state)]. Based on this fact, some proposals have been made to control the molding process by the PVT property, such as described on the pages 11 and 12 of Modern Plastics International, October 1989. This PVT property-based control method also has the following problems:
(i) To control the molding process by the PVT property, the PVT property must be given or determined in advance for practical use. Determination of the PVT property requires accurate measurement using a special measurement instrument in a test facility.
(ii) Various kinds of plasticized synthetic resins are used in actual injection molding and, in many cases, new plasticized synthetic resins are introduced one after another. Therefore, if the PVT property of certain type of plasticized synthetic resin can be obtained experimentally to provide the experimental determination of the PVT property of all the plasticized synthetic resins to be used would involve enormous time and expense and is not feasible.
(iii) Moreover, for the same plasticized synthetic resin, the experimentally determined PVT property does not always coincide with the PVT property in an injection molder actually used at the injection molding site because of the following reason. In an experimental equipment, the plasticized synthetic resin is heated only externally. In an actual injection molder, in contrast, the thermal history and extremely large shearing force during the melting process of the plasticized synthetic resin cause slight change in the molecular distribution of the resin. Consequently, the melting behavior of the resin in the actual injection molder may be different from that in the experimental equipment.
(iv) Furthermore, for recycled use of waste resin material, the actual injection molding may use reclaimed plasticized synthetic resin by mixing new plasticized synthetic resin material. Even if new plasticized synthetic resin pellets are used, the quality of the pellets may vary among different production lots or even in the same production lot. Accordingly, it is very difficult to apply the experimentally obtained PVT property directly to the actual injection molding.
The molten resin temperature (T) as the PVT property, or the molten state such as flowability of resin indicated by the molten resin temperature (T) can also be indicated by the revolution or back-pressure of the screw, the time required for measuring the molten resin in the actual injection molding, or a combination of some of these three factors. Therefore, the PVT property can be generalized by replacing the molten resin temperature (T) with the molten state (Z).
It is an object of the present invention to solve the above problems in the resin property-based molding process control method. Specifically, an object of the present invention is:
(i) to provide a method for detecting the resin property of plasticized synthetic resin, which is capable of:
1. Detecting easily the resin property of the plasticized synthetic resin presently used in injection molders at the actual injection molding site, and PA1 2. utilizing the thus detected resin property directly for controlling the injection molding process under way in the site, and PA1 V.sub.o : molten resin specific volume at reference molten resin pressure value P.sub.o PA1 P.sub.o : reference molten resin pressure value (reference pushing force value p.sub.o) PA1 P: molten resin pressure value (pushing force value p) PA1 a(Z): constant determined by the molten state value PA1 i) When the molten resin volume value remaining between the screw end and the block valve is given as a design value: PA1 ii) When the molten resin volume value remaining between the screw end and the block valve is unknown: PA1 (a) the first process comprising the steps of closing the flow path opening/closing mechanism to block the resin flow, and under the blocked state of the resin flow, applying various pushing force values p to the screw so as to effect the balancing movement of the screw, and obtaining the positional value of the screw at each first stop position where the screw stops its balancing movement under each pushing force value p, PA1 (b) the second process comprising the steps of opening the flow path opening/closing mechanism to allow the resin to flow, and under the blocked state of the resin flow, applying a pushing force to the screw so as to permit plasticized synthetic resin of an appropriate weight value G to be injected, and PA1 (c) the third process comprising the steps of closing the flow path opening/closing mechanism to block the resin flow after the injection of the resin of the weight value G, applying the various pushing force values p of the same values as those for the first process to the screw so as to effect the balancing movement of the screw, and obtaining the positional value of the screw at each second stop position where the screw stops its balancing movement under each pushing force value p; PA1 S.sub.H : positional value of the screw during the dwelling process following the injection, for the molten state value Z PA1 S.sub.I : positional value of the screw immediately before the injection, for the molten state value Z PA1 G: weight value of a target product PA1 A: projected sectional area of the screw PA1 V(P.sub.H,Z): molten resin specific volume value for the given molten state value Z and the molten resin pressure value P.sub.H during the dwelling process following the injection, for the molten state value Z PA1 V(P.sub.I,Z): molten resin specific volume value for the given molten state value Z and the molten resin pressure value P.sub.I immediately before the injection, for the molten state value Z
(ii) to provide an injection control method capable of calculating an appropriate screw travel distance on the basis of the resin property detected by the above-mentioned detection method by using a given calculation formula, thereby controlling the weight of plasticized synthetic resin injected from the cylinder of the injection molder to fill the cavity of a mold.